The present invention relates to acoustic signal processing systems, and more particularly to method and apparatus for enhancing a nonrandom underwater acoustic signal against the ambient, random background noise existing in a sea environment.
As is well known the sound waveforms generated by a man-made object, such as a submarine, having the characteristics of being relatively constant with time and of low variance, i.e., generally not normally distributed either in frequency or intensity when measured over short intervals. In contrast the background noise existing naturally in a sea environment is typically high random having large variances both in intensity and frequency content. In fact it can be postulated that given a sufficiently large volume of ocean the sum of the noise generated therein will closely approximate normal or Gaussian characteristics in the absence of any man-made noise. This postulate is possible even if individual noise generators existing naturally in the ocean such as fish are each characterized by nonnormal distributions, since when they are cumulated the distribution of their nonnormality is random and consequently their sum is normal. Thus, on the basis of this postulate it can be concluded that by integrating with time the total sound generated naturally in the ocean the integral will tend to converge on zero while the nonnormal or nonrandom man-made sound will tend to accumulate a nonzero value. Various techniques in the prior art have taken advantage of this fact in providing data reduction techniques identifying an azimuth of a man-made signal against the random background of the sea by integrating with time the sound emanated along a given vector thus providing the means of searching out the azimuth position of the man-made source. Some of these techniques additionally break the sound up into its spectral components providing even more signal enhancement in view of the tendency of man-made sources to have distinct spectral signatures. In this manner a very low signal-to-noise ratio can be processed in order to identify and locate an object like a submarine within a large volume of ocean.
The above techniques, however, are limited in their resolution due to the existence of low intensity sidebands usually accompanying any high intensity narrow-band source, which can locally combine with nonnormal sources existing naturally in the sea such that, over extended integration, false signature and target coordinates are produced. Specifically, it is the non-Gaussian character of this broadband noise emanated by a submarine which can bias the integral of the noise background, thereby resulting in a nonzero integral that is limiting the resolution of the signature. This broadband noise is generally insufficient in intensity to be resolved against the background, even over extended integration intervals, consequently it can combine on either side of a given azimuth with short period nonrandom noise existing naturally forming a false azimuth location.